A novel and reliable approach for controlling silicon membrane thickness with smooth surface

Abstract

Despite self-supporting ultrathin Si membranes are already being widely applied, its thickness control in micron scale still remains a big challenge for delicate systems such as micro-opto-electro-mechanical systems and silicon-based X-ray diffractive optics. Instead of using the conventional wet etch rate of Si to monitor the membrane thickness, this work has developed a reliable approach to control the membrane thickness in the precision of micrometer order by using an optical detection hole with high precision depth, which decides the membrane thickness in the Si thinning process. The wet etch rate is first optimized to ensure the fundamental condition for such a thickness control. Furthermore, the membrane surface roughness is greatly reduced by optimizing the additive ethanol concentration in the KOH etchant, based on the investigation results for the origin of the membrane surface roughness. Using the proposed etching and thickness-monitoring method, self-supporting micron thick Si (100) membranes with the surface roughness of <15 nm have been successfully fabricated.